Connection memory for tributary time-space switches

ABSTRACT

A method of switching a plurality of tributaries disposed among a plurality of time slots in a frame is disclosed. The method generally includes the steps of (A) buffering the frame, (B) switching the tributaries among the time slots in response to a read address and (C) generating the read address in response to a plurality of identifications in a connection map, the connection map defining (i) at most one of the identifications for each of the tributaries and (ii) one of the identifications for each of the time slots carrying other than the tributaries.

FIELD OF THE INVENTION

The present invention relates to time-space switches generally and, moreparticularly, to a connection memory for tributary time-space switches.

BACKGROUND OF THE INVENTION

Transport network standards, such as the Synchronous Optical Network(SONET) and the Synchronous Digital Hierarchy (SDH), are used in timedivision multiplexed (TDM) networks in which link capacity is evenlydivided temporally for efficient bandwidth management. The lowestbandwidth, or most granular, “high-order” switching unit of a SONETframe is a Synchronous Transport Signal, level-1 (STS-1) frame. EachSTS-1 frame comprises nine rows of 90-columns transmitted in 125microseconds (μs). As such, the STS-1 frame rate is 51.84 million bitsper second (Mbps). Multiple SONET STS-1 frames can be multiplexedtogether to form higher rate frames. Currently, the defined SONET framerates are STS-1, STS-3, STS-12, STS-48, STS-192, and STS-768. Some SONETframe designations have an appended “c” that indicates payloadconcatenation. An STS-N frame and a STS-Nc frame have the same framerate, where N is 1, 3, 12, 48, 192, or 768.

In SDH, a Synchronous Transport Module, level-0 (STM-0) frame has thesame frame rate and row-column structure as the SONET STS-1 frame.Higher levels of SDH frames are known as STM-N frames, where N can be 1,4, 16, 64, and 256, corresponding to the same frame rates and row-columnstructures as SONET STS-3c, STS-12c, STS-48c, STS-192c, and STS-768c,respectively, as illustrated in TABLE I. The implementation of anSTS-1/STM-0 time-space switch, also known as a SONET/SDH column switch,thus can treat the SONET columns and the SDH columns similarly. TABLE IFrame Format Rate SONET Frame SDH Frame Rows Columns (Kbps) STS-1 STM-09 90 51,840 STS-3c STM-1 9 270 155,520 STS-12c STM-4 9 1,080 622,080STS-48c STM-16 9 4,320 2,488,320 STS-192c STM-64 9 17,280 9,953,280STS-768c STM-256 9 69,120 39,813,120

A time-space switch with a lower level of switching granularity thanSTS-1/STM-0, however, may have to implement logic that distinguishes aSONET frame from an SDH frame. Each SONET STS-1 frame carries a payloadin the synchronous payload envelope (SPE), which in turns carries“low-order” switching units know as virtual tributaries (VTs). In SDH,the low-order switching units are known as tributary units (TUs). TABLEII summarizes the frame sizes and rates for the virtual tributaries asfollows: TABLE II SONET Virtual SDH Frame Tributary Tributary FormatRate (VT) Unit (TU) Rows Columns (Kbps) VT1.5 TU-11 9 3 1,728 VT2 TU-129 4 2,304 VT3 — 9 6 3,456 VT6 TU-2 9 12 6,912

Existing tributary time-space switches use the following solutions: (1)a full-blown column switch and (ii) limited data formats. The full-blowncolumn switch means that any SONET/SDH column can be switched to anyother column. The full-blown column switch is costly to implement insilicon. The limited data format approach limits the arrangementsallowed in a frame to accommodate the switching. Some conventionalswitches support the North American standard only (SONET) but not theEuropean and Asian standard (SDH). Some conventional switches preformatincoming data to a supported format. Some conventional switches canprocess VT1.5 but not VT2 traffic, although both are part of the SONETstandard.

SUMMARY OF THE INVENTION

The present invention concerns a method of switching a plurality oftributaries disposed among a plurality of time slots in a frame. Themethod generally comprises the steps of (A) buffering the frame, (B)switching the tributaries among the time slots in response to a readaddress and (C) generating the read address in response to a pluralityof identifications in a connection map, the connection map defining (i)at most one of the identifications for each of the tributaries and (ii)one of the identifications for each of the time slots carrying otherthan the tributaries.

The objects, features and advantages of the present invention includeproviding a connection memory for tributary time-space switches that may(i) consume less memory space than a conventional connection memory,(ii) process all tributary standards defined in SONET, (iii) process alltributary standards defined in SDH, (iv) maximize switching bandwidth ofa tributary time-space switch given a fixed amount of silicon area, (v)operate without incoming data preformatted to a particular format, (vi)use about ⅓rd of the connection memory bits compared with a conventionalcolumn switch and/or (vii) support column override.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andthe appended claims and drawings in which:

FIG. 1 is a diagram of an example virtual tributary structures in aSynchronous Optical Network frame in accordance with a preferredembodiment of the present invention;

FIG. 2 is a diagram of example tributary unit structures in aSynchronous Digital Hierarchy frame;

FIG. 3 is a block diagram of an example implementation of a firstsystem;

FIG. 4 is a block diagram of an example implementation of a secondsystem comprising multiple switch core circuits;

FIG. 5 is a block diagram of a connection map for the second system;

FIG. 6 is a block diagram of an example implementation of a SONETtributary switching system;

FIG. 7 is a diagram of an exemplary unaligned STS-1 frame and aresulting aligned STS-1 frame;

FIG. 8 is a table of an example tributary switching unitidentifications;

FIG. 9 is a table of a first example data field in a tributary typememory circuit;

FIG. 10 is a table of a second example data field in the tributary typememory circuit;

FIG. 11 is a diagram of a VC-4 structured STM-1 frame;

FIG. 12 is a table of a third example data field in the tributary typememory circuit;

FIG. 13 is a table of a fourth example data field in the tributary typememory circuit; and

FIG. 14 is a diagram of an example implementation of a column overridefunction in a system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a diagram of an example virtual tributary (VT)structures in a Synchronous Optical Network (SONET) frame 100 is shownin accordance with a preferred embodiment of the present invention. Theframe 100 is generally illustrated as a SONET Synchronous TransportSignal, level-1 (STS-1) frame. The SONET frame 100 generally comprisesmultiple (e.g., 90) columns (labeled left to right as columns 1-90).Each of the columns generally comprises multiple (e.g., 9) rows. Eachrow may have 90 bytes of data. The virtual tributaries may include VT1.5tributaries, VT2 tributaries, VT3 tributaries and VT6 tributaries.

The first three columns in the 90-column STS-1 frame 100 are generallydefined as transport overhead (TOH) columns 102. A path overhead (POH)column may be anywhere in a synchronous payload envelope (SPE) of theframe 100. The SPE generally comprises the rows from column 4 to column90. For purposes of discussion, the POH column may be assumed to residein column 4. Columns 33 and 62 may be referred to as fixed stuff (e.g.,S1 and S2) columns and do not belong to any particular VT.

Each STS-1 frame 100 may carry seven VT groups 104 a-104 g (only VTgroups 104 a-104 d are shown for clarity). The VT groups 104 a-104 g maybe generically and individually referred to as a VT group 104. Each VTgroup 104 may occupy twelve columns and carry virtual tributaries (or“tributaries” for short) of the same type. Possible combinations that aVT group 104 may carry include, but is not limited to, four VT1.5tributaries, three VT2 tributaries, two VT3 tributaries or one VT6tributary. FIG. 1 generally illustrates mapping of the VT groups 104a-104 g to into the frame 100.

Each VT group 104 generally comprises one or more individual virtualtributaries 106 a-106 z (only some individual VTs are shown forclarity). The VTs 106a-106 z may be generically and individuallyreferred to as a VT 106. A number of columns in each of the VTs 106 aregenerally provided in TABLE II above. In the example of FIG. 1, fourindividual VT1.5s 106 a-106 d may be disposed in the VT group 104 a.Three VT2s 106x-106 z may be disposed in the VT group 104 b. Two VT3s106 m-106 n may be disposed in the VT group 104 c. One VT6 106 p may bedisposed in the VT group 104 d. Other arrangements of VTs 106 and VTgroups 104 may be implemented to meet the criteria of a particularapplication.

Referring to FIG. 2, a diagram of an example tributary unit (TU)structures in a Synchronous Digital Hierarchy (SDH) frame 110 is shown.The frame 110 is generally illustrated as an SDH Synchronous TransportModule, level-0 (STM-0) frame. The SDH frame 110 generally comprisesmultiple (e.g., 90) columns (labeled left to right as columns 1-90).Each of the columns generally comprises multiple (e.g., 9) rows. Eachrow may have 90 bytes of data. The tributaries units may include TU-11tributaries, TU-12 tributaries and TU-2 tributaries.

The tributary units are generally the SDH counterparts of the SONET VTs.According to the SDH standard, the TUs may be addressed much like VTs,with the exception that the fixed stuff columns may occupy either (i)columns 33 and 62 or (ii) columns 5 and 6 in an STM-0 frame. Inparticular, a tributary group 2 (e.g., TUG-2) in SDH may be similar to aVT group 104 in SONET. The above observations enable the VT/TUconnectivity information to be compressed in a connection map of atributary time-space switch circuit while allowing mixing both SONET VTsand SDH TUs in one switch.

The first three columns in the 90-column STM-0 frame 110 are generallydefined as the transport overhead (TOH) columns 102. The path overhead(POH) column may be anywhere in a synchronous payload envelope (SPE) ofthe frame 100. The SPE generally comprises the rows from column 4 tocolumn 90. For purposes of discussion, the POH column may be assumed toreside in column 4. Columns 33 and 62 or columns 5 and 6 may be referredto as the fixed stuff S1 and S2 columns and do not belong to anyparticular TU.

Each STM-0 frame 110 may carry seven TU groups 114 a-114 g (only TUgroups 114a-114 c are shown for clarity). The TU groups 114 a-114 g maybe generically and individually referred as a TU group 114. Each TUgroup 114 may occupy twelve columns and carry tributary units (or“tributaries” for short) of the same type. Possible combinations that aTU group 114 may carry include four TU-11 tributaries, three TU-12tributaries or one TU-2 tributary. FIG. 2 generally illustrates mappingof the TU groups 114 to into the frame 110.

Each TU group 114 generally comprises one or more individual tributaryunits 116 a-116 z (only some individual TUs are shown for clarity). TheTUs 116 a-116 z may be generically and individually referred to as a TU116. A number of columns in each of the TUs 116 are generally providedin TABLE II above. In the example of FIG. 2, four individual TU-11s 116a-116 d may be disposed in the TU group 114 a. Three TU-12s 116 x-116 zmay be disposed in the TU group 114 b. One TU-2 116 p may be disposed inthe TU group 114 c. Other arrangements of TUs 116 and TU groups 114 maybe implemented to meet the criteria of a particular application.

Being able to cost-effectively manage bandwidth at the tributary levelis generally more advantageous to service providers than at the lessgranular STS-1/STM-0 frame level. For instance, T1 traffic may be mappedonto one VT1.5 and an STS-1 payload may carry up to 28 such tributaries(or streams). Similarly, a 10BaseT Ethernet connection (10 Mbps) may bemapped onto seven VT1.5s and an STS-1 may therefore carry 4 suchconnections. Tributary time-space switching thus increases bandwidthutilization compared with bandwidth management at the STS-1/STM-0 level.

To simplify system design, a tributary time-space switch of the presentinvention may be able to switch any mix of the tributaries listed inTABLE II. Without such a capability in the tributary time-space switchthe system would, for instance, have to convert one tributary format toanother solely for switching purposes. For instance, the system mightconvert a VT1.5 into a VT2 by stuffing the VT1.5 and passing theresulting VT2 to a tributary time-space switch capable of switching VT2sonly. Similarly, a tributary time-space switch may also be able toswitch SONET and SDH tributaries.

Referring to FIG. 3, a block diagram of an example implementation of afirst system 120 is shown. The system (or circuit) 120 may be referredto as a tributary time-space switch, or switch for short. The switch 120may be configured to groom traffic from a number (e.g., P) STS-N streams(where N is 1, 3, 12, 48, 192, or 768) with column granularity in anunrestricted non-blocking fashion. As such, some of the input columnsmay be switched in both space and time.

The time-space switch 120 generally comprises a switch core circuit (ormodule) 122, a memory circuit (or module) 128 and a controller circuit(or module) 130. A signal (e.g., IN) may be received by the switch corecircuit 122 at an input port 132. A signal (e.g., OUT) may be presentedat an output port 134 of the switch core circuit 122. A signal (e.g.,SCWA) may be presented from the controller circuit 130 to the switchcore circuit 122. A signal (e.g., CMRA) may be presented from thecontroller circuit 130 to the memory circuit 128. A signal (e.g., SCRA)may be presented from the memory circuit 128 to the switch core circuit122.

The signal SCWA may be referred to as a switch core write addresssignal. The signal SCWA may carry write address values to the switchcore circuit 122 identifying where to write the individual columns of anincoming frame 100/110 in the signal IN.

The signal SCRA may be referred to as a switch core read address signal.The signal SCRA may carry read address values to the switch core circuit122 identifying a particular one of the individual columns of a storedframe 100/110 to read via the signal OUT.

The signal CMRA may be referred to as a connection map read addresssignal (or a counter signal). The signal CMRA may carry read addresses(or a column counter) to the memory circuit 128 identifying a particularconnection map value to be used in generating the read address in thesignal SCRA.

The switch core circuit 122 generally comprises a buffer circuit (ormodule) 124 and a buffer circuit (or module) 126. The buffer circuits124 and 126 may be similar to each other. Each of the buffer circuits124 and 126 may be capable of storing P rows of STS-N data (e.g., 90PNbytes). Each buffer circuit 124 and 126 generally alternates betweenbeing a read buffer and a write buffer for 90N byte cycles, where onebuffer may be the read buffer while the other one may be the writebuffer. To achieve the time-space switching function, as data from acurrent row is filling the write buffer sequentially per the signalSCWA, data from a previous row may be read from the read bufferaccording to an order specified in the signal SCRA.

The memory circuit 128 may be configured to store a Tributary TypeMemory (TTMEM)(or module) 136 and a connection map (or module) 138. TheTTMEM 136 and the connection map 138 generally serve as a generator forthe switch core read address values in the signal SCRA. The switch coreread addresses, in turn, is generally indexed by the output columncounter addresses generated by the controller circuit 130 in the signalCMRA. The TTMEM 136 generally stores tributary type information used inrouting the VTs and TUs. The connection map 138 generally storesswitching information also used in routing the VTs and TUs.

The switch core circuit 122, generally has P input (write) ports 132 andQ output (read) ports 134. The P input ports 132 may be logicallyconfigured as a single write port of 8P bits wide. The Q output ports134 may be logically configured as Q read ports, each 8 bits wide.

Physical implementation issues may limit the number of read ports 134 ofeach switch core circuit 122 to have fewer than Q read ports 134. Forexample, each switch core circuit 122 may have K read ports 134.Therefore, M=┌Q/K┐ copies of the K-read-port-one-write-port switch corecircuit 122 may be implemented to produce Q read ports 134. The function┌x┐ generally returns a smallest integer greater than or equal to x.

Referring to FIG. 4, a block diagram of an example implementation of asecond system 150 comprising multiple switch core circuits 122 a-122 mis shown. The system (or circuit) 150 generally comprises M switch corecircuits 122 a-122 m, M memory circuits 128 and the controller circuit130, where M is an integer greater than one. The switch core circuits122 a-122 m may be similar to the switch core circuits 122, only with Kread ports, where K<Q and MK=Q.

The switch core circuits 122 a-122 m are generally illustrated with apage from each of the buffer circuits 124 and 126. The other pages ofthe buffer circuits 124 and 126 are not shown for clarity. Each of thetwo pages of the same buffer circuit 124/126 generally share the same Kread addresses and read data ports. Each copy of the switch core circuit122 a-122 m is generally indexed by the connection map 138 (see FIG. 3).The TTMEM 136 and the connection map 138 may be used by the memorycircuit 128 to generate K independent read addresses. All M copies ofthe memory circuits 128 generally share the same read address from thecontroller circuit 130. The read address is typically a counterindicating a current output time slot. Conventionally, each switch corecircuit stores 90×8×2×P×N=1440PN bits of data from the received frames100/110. The M switch core circuits may store 1440MPN bits of data fromthe received frames 100/110. For instance, to accommodate an 80 billionbits per second (Gbps) SONET tributary, a time-space switch having P=32and STS-48 (N=48) input ports may store M×2,211,840 bits.

Referring to FIG. 5, a block diagram of a connection map 160 for thesecond system 150 is shown. The connection map 160 generally comprisesmultiple (e.g., Q) connection memory modules 162 a-162 q (Q instances ofmodule 162 a), one for each one of the Q output ports of the system 150.The connection memory modules 162 a-162 q may be individually referredto as a connection memory module 162. The read address of a connectionmemory module 162 for a particular output port generally corresponds tothe output time slot for that port. For the system 150, 90N output timeslots may exist for each output port and the controller circuit 130 maycycle through all 90N read addresses, one address per output time slot,repeatedly wrapping around to the beginning. To support hitlessreconfiguration, whereby all SONET/SDH frames may be switched intactbefore and after the reconfiguration event, the connection memorymodules 162 a-162 q may be double-buffered. Therefore, a reconfigurationevent that sends part of a SONET/SDH frame 100/110 to one output timeslot before the reconfiguration event and the rest of the frame 100/110to a different output time slot after the event is not hitless. Half ofthe connection memory modules 162 a-162 q may be active while the otherhalf may be on standby. While the controller circuit 130 reads theactive connection map, a new set of connections may be substantiallysimultaneously written to the standby connection map. The controllercircuit 130 generally swaps the active map and the standby map at theSONET/SDH frame boundary to achieve hitless reconfiguration. Thus, aconventional connection memory map may have 180N entries.

The output read address generated from the above connection memory map160 may provide an input to a function generating the read address valuefor the switch core circuit 122. For the above system 150, 90PNaddresses may exist, corresponding to 90PN input time slots to which anoutput time slot may be connected. The output data of the conventionalconnection memory modules are thus at least ┌log₂90PN┐ bits wide. Thetotal number of bits in the conventional connection map that has Qconnection memory modules may thus be 180QN┌log₂90PN┐ bits.

Referring to FIG. 6, a block diagram of an example implementation of aSONET tributary switching system 170 is shown. The system (or circuit)170 generally comprises multiple circuits (or modules) 172 a-172 n and acircuit (or module) 174. The circuit 174 may be operational as atributary switch core circuit. The tributary switch core circuit 174 maybe similar to either the switch core circuit 122 or the switch corecircuits 122 a-122 m.

The circuits 172 a-172 n may be referred to as column aligner circuits.The column aligner circuits 172 a-172 n may be operational to pre-alignthe synchronous payload envelope (SPE) in each STS-1/STM-0 frame beforepassing the SONET/SDH frame to the tributary switch core circuit 174.The exact location of the first byte of the SPE is generally immaterialsince the first byte is the same among all SPE-aligned STS-1/STM-0frames.

Referring to FIG. 7, a diagram of an exemplary unaligned STS-1 frame 180and a resulting aligned STS-1 frame 182 are shown. The unaligned STS-1frame 180 may have (i) the transport overhead (TOH) columns 184 (e.g.,the first three columns of the frame) aligned as normal and (ii) thepath overhead (POH) column 186 and other columns within the SPE 188unaligned with an envelope capacity 190 (e.g., column 4 through column90) inside the unaligned frame 180. The column aligner circuits 172a-172 n may realign the columns in the envelope capacity 190 of theunaligned frame 180 such that the entire SPE 188 aligns with theenvelope capacity 190 of the aligned frame 182. As such, the POH column186 generally occupies the fourth column in the aligned frame 182.

Since each STS-1 frame has at most 28 tributaries (e.g., 28 VT1.5s), thetributaries may be interleaved in time and the temporal ordering of thetributary columns within a tributary payload may be fixed. For example,the tributary time-space switch circuit generally maintains the temporalordering of the four columns within a particular VT2. Therefore, storingan entire row of a SONET frame in one page of the switch core circuit122 before letting the controller circuit 130 read the data out may beunnecessary. While a conventional column switch core circuit isconceptually simple, the conventional switch core circuit providesflexibility that the SONET tributary time-space switch may not utilize,namely an ability to switch any input column to any output column.Instead, the present invention provides techniques that exploit SONETtributary switching patterns to lower memory usage in the connection map138.

The present invention generally provides a memory-efficient connectionmap for tributary time-space switches. A tributary time-space switch,being a connection-oriented switch and comprising a plurality of inputsand a plurality of outputs, is capable of switching SONET/SDH tributarypayloads both in space and in time according to the connectivityinformation stored in the connection map. According to one embodiment ofthe invention, a connection map comprising a plurality of memory modulescontaining compressed switching connectivity information generallyreduces memory usage compared with conventional designs.

Referring again to FIG. 1, the tributary time-space switch 120/150/170may maintain a temporal ordering of the columns within a tributary(e.g., VT or TU). For instance, the tributary time-space switch mayprohibit (e.g., arrow 191) time-switching column 5 (e.g., column A1 inthe first column of the VT 106 a and in the first column of the VT group104 a) in the STS-1 frame 100 with column 34 (e.g., the column A2 in thesecond column of the VT 106 a and in the fifth column of the VT group104 a) in either the same or another STS-1 frame. The prohibitiongenerally prevents temporal reordering the columns belonging to aparticular tributary regardless if the VT Group 104 carries only VT1.5s,VT2s, VT3s or a VT6s. A similar prohibition may be applied to the STM-0frame 110.

The tributary time-space switch 120/150/170 may allow (e.g., arrows 192)time-switching columns within a particular group 104/114 where thetemporal ordering among the switched columns remains unchanged. Forexample, columns A1, A2 and A3 in the tributary 106 a may be switchedrespectively into columns B1, B2 and B3 of the same tributary 106 a.Similarly allowed time-switches may be applied to the STM-0 frame 110.As such, the connection map 138 may only store sufficient data to switcha single column of a particular tributary. The tributary time-spaceswitch may apply the data for the single column to all other columnswithin the particular tributary to maintain temporal ordering. Spendingadditional memory to specify where to switch the other columns in theparticular tributary may be redundant and thus avoided.

Referring again to FIG. 2, the tributary time-space switch 120/150/170may also allow (e.g., arrows 193) time-switching columns from a firstgroup 104/114 to a second group 104/114 where the temporal orderingamong the switched columns remains unchanged. For example, the columnsA1, A2 and A3 in the TU group 114 a may be switched respectively intocolumns Z1, X3 (four columns right of Z1) and Y4 (four columns right ofX3) in the TU group 114 b. Similarly allowed time-switches may beapplied to the STS-1 frame 100.

Referring to FIG. 8, a TABLE III of an example tributary switching unitidentifications (IDs) is shown. Rather than addressing each of the 90columns individually in an STS-1/STM-0 frame when switching VTs/TUs, thepresent invention addresses at most 34 VT/TU switching units defined inTABLE III to completely address all VTs and TUs defined in the SONET andSDH standards. Addressing 34 VT/TU switching units, as opposed toconventionally addressing 90 columns, in an STS-1/STM-0 frame generallyreduces the connection map 138 memory bit usage from 180QN┌log₂90PN┐bits to 68Q┌log₂34PN┐ bits. Letting T be a number of unique VT/TUswitching unit IDs, the memory bit usage of the connection map 138 maybe given as 2TQN┌log₂TPN┐ bits. The same VT/TU switching unit ID may beused for both SONET and SDH. However, the switch core address values aregenerally different between SONET and SDH frames. As such, the type ofthe VT/TU switching unit being manipulated should be identified.

Referring to FIG. 9, a TABLE IV of a first example data field in theTTMEM 136 is shown. The TABLE IV generally summarizes for eachSTS-1/STM-0 frame whether the frame is (i) a VT-structured SONET frame,(ii) a VC-3 structured SDH frame or (iii) a VC-4 structured SDH frame.For an STS-N frame, or equivalent frame, each output port generallycontains one TTMEM 136 with 2N bytes. The factor of two supports doublebuffering for hitless reconfiguration.

Referring to FIG. 10, a TABLE V of a second example data field in theTTMEM 136 is shown. The TABLE V may be used to compress the connectionmap information further than TABLE IV. Since the fixed stuff columns arein either (i) columns 5 and 6 or (ii) columns 32 and 62, the stuffcolumn information may be encoded in the TTMEM 136 and the tributarytime-space switch prohibited from cross-connecting fixed stuff columns.As such, the output time slots corresponding to fixed stuff columns asspecified in the TTMEM 136 may only connect to input time slotscontaining fixed stuff columns. The stuffed columns generally result inthe input frame and the output frame to be similarly structured as theinput and the output stuffed columns generally occupy the same timeslots within a VC-3 or VC-4 frame. The number of bits in the connectionmap 138 generally decreases from 180QN┌log₂90PN┐ bits in theconventional approach to 64QN┌log₂32PN┐ bits (e.g., T=32).

Referring to FIG. 11, a diagram of a VC-4 structured STM-1 frame isshown. The SDH generally specifies that four TU-11s, three TU-12s, orone TU-2 may be multiplexed into a TUG-2, seven of which may bemultiplexed into either (i) a VC-3, which in turn may go into an(Administrative Unit) AU-3 or (ii) a TUG-3, three of which in turn maybe multiplexed into a VC-4, which may go into an AU-4. In addition, oneTU-3 may go into one TUG-3, three of which may be multiplexed into aVC-4, which then may go into an AU-4. Three AU-3s or one AU-4 may thengo into an (Administrative Unit Group) AUG-1. In all cases, if the AUG-1contains an AU-3, then the stuffed columns may occupy columns 33 and 62using the equivalent SONET STS-1 column numbering in FIG. 1; otherwise,the AUG-1 contains an AU-4, in which case, the stuffed columns mayoccupy columns 5 and 6 (as shown in FIG. 11).

Referring to FIG. 12, a TABLE VI of a third example data field in theTTMEM 136 is shown. The TABLE VI data may further compresses theconnection map 138 by specifying the three transport overhead (TOH)columns 102 as a single VT/TU switch unit ID. The three columns may bewell defined in the SONET and SDH standards and time-switching withinthe three columns may not be performed. The further enhancement bringsthe number of connection map 138 bits down to 60QN┌log₂30PN┐ bits (e.g.,T=30)

Referring to FIG. 13, a TABLE VII of a fourth example data field in theTTMEM 136 is shown. As long as the path overhead (POH) columns arepre-aligned to the same time slot, assumed without loss of generality tobe column 4, the POH and the TOH columns may be grouped together as asingle VT/TU switch unit ID. Therefore, the connection map 138 may befurther lowered to 58QN┌log₂29PN┐ bits (e.g., T=29).

Keeping T at or less than 32 generally prevents a width of theconnection map 138 from growing an extra bit. The above discussion maybring T down to 29 by imposing constraints generally consistent with theSONET and SDH standards. Three extra VT/TU switching unit IDs may beavailable when T is reduced to 29 before the total number of uniqueVT/TU switching unit IDs reaches 32 (e.g., a 5-bit value). The extraVT/TU switching unit IDs may be used to signal other actions for thetributary time-space switch to perform. For example, one or more of theextra switching unit IDs may be defined as override indications.

Referring to FIG. 14, a diagram of an example implementation of a columnoverride function in a system 200 is shown. The system (or circuit) 200generally comprises the switch core circuit 122, the memory circuit 128,a circuit (or module) and a multiplexer 204. The system 200 may besimilar to the system 120 (FIG. 3) with the added circuit 202 and theadded multiplexer 204. The tributary type information in TABLE IV may bestored, in the memory circuit 128.

The circuit 202 may be referred to as a logic circuit. The logic circuit202 may be operational to generate a signal (e.g., A) in response to thesignal SCRA. The signal A may carry a bit pattern a single column wide.The logic circuit 202 may also be operational to generate a signal(e.g., S) in response to the signal SCRA. The signal S may be referredto as a selection signal used to control the multiplexer 204.

When the VT/TU switching unit ID in the signal SCRA is one of theoverride IDs, the logic circuit 202 may generate the selection signal Son a link 208 such that the data coming out of the switch core circuit122 in the data signal D on a link 206 may be ignored by the 2-to-1multiplexer 204. Instead, the logic circuit 202 may generate anassociated override byte in the signal A on a link 210. As such, theframe in the output signal OUT may convey one or more override patternsfrom the signal A in place of one or more columns of data received inthe signal IN.

The extra IDs may be used for generating column overrides. The columnoverrides may be useful for encoding alarms and provisioning informationinto the outgoing stream of the signal OUT. For instance the threeswitching unit IDs for column overrides may define (i) an alarm, (ii) anunequipped indication and (iii) a predefined constant. The alarmcondition may be indicated by a first predetermined pattern (e.g., allones, hexadecimal FF). The unequipped condition may be used to indicatethat the particular column (or time slot) being overridden isunequipped. The predefined constant may be used for user definedsignaling purposes.

The present invention may implement SONET/SDH tributary addressing byfirst classifying the switching unit (set of columns in a SONET/SDHframe) into a transport overhead column (TOH), a path overhead column(POH), a fixed stuff column or a VT/TU. If the latter, reception of anSTS-1 frame or an STM-0 frame may be identified (a particular column outof every N columns starting with the first column of an STS-N frame oran STM-N/3 frame for N no less than 3). The VT Group/TUG-2 may then beidentified. Individual VTs/TUs may also be identified. The frame, groupand tributary information may be encoded in a compressed connection maphaving a size no more than TQN┌log₂TPN┐ bits, where T is substantiallybelow 90, by encoding locations of switching units (VTs and TUs) ratherthan individual columns in a SONET/SDH frame.

The structure of each STS-1/STM-0 frame may be identified to aiddecoding of the compressed connection map with a decoder memory (alsoreferred to as the TTMEM) of size 2N bytes to enable the connection mapto encode locations of tributaries in a SONET/SDH frame, rather thanaddressing individual columns of each tributary. Location of fixed stuffcolumns may be encoded in the information stored in the TTMEM. Thesystem may prohibit fixed stuff cross-connections. The fixed stuffcolumns from the input frame may be in the same place as in the outputframe. As such, both the input frame and output frame may be similarlystructured tributary SONET/SDH frames.

Entries may be added to the connection map to support column override toencode special signaling conditions such as (i) SONET/SDH path alarmindication signal, (ii) VT tributary alarm indication signal (AIS-V) and(iii) an unequipped path. Given additional conditions to be encoded inthe connection map, the connection map size may be given as2(T+C)QN┌log₂(T+C)PN┐ bits, where C may represent a number of thesignaling conditions.

The transport overhead (TOH) columns may be switched as a single groupof columns instead of three separate columns. The POH column may beswitched separately from the TOH columns. Furthermore, the POH columnand the TOH columns may be treated as a single group for switchingpurposes.

In one embodiment, the system may provide separate memory circuits forstoring the compressed connection information (e.g., connection map 138)and the tributary type information (e.g., TTMEM 136). In anotherembodiment, both the compressed connection map data and the tributarytype information may be stored in a single memory circuit (e.g., 128).The TTMEM information may also be put inside a compressed connectionmap.

The various signals of the present invention are generally “on” (e.g., adigital HIGH, or 1) or “of” (e.g., a digital LOW, or 0). However, theparticular polarities of the on (e.g., asserted) and off (e.g.,de-asserted) states of the signals may be adjusted (e.g., reversed)accordingly to meet the design criteria of a particular implementation.Additionally, inverters may be added to change a particular polarity ofthe signals. The present invention may also be implemented by thepreparation of ASICs, FPGAs, or by interconnecting an appropriatenetwork of conventional component circuits (such as conventional circuitimplementing a state machine), as is described herein, modifications ofwhich will be readily apparent to those skilled in the art(s). As usedherein, the term “simultaneously” is meant to describe events that sharesome common time period but the term is not meant to be limited toevents that begin at the same point in time, end at the same point intime, or have the same duration.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A method of switching a plurality of tributaries disposed among aplurality of time slots in a frame, comprising the steps of: (A)buffering said frame; (B) switching said tributaries among said timeslots in response to a read address; and (C) generating said readaddress in response to a plurality of identifications in a connectionmap, said connection map defining (i) at most one of saididentifications for each of said tributaries and (ii) one of saididentifications for each of said time slots carrying other than saidtributaries.
 2. The method according to claim 1, wherein said framecomprises one of a Synchronous Optical Network frame and a SynchronousDigital Hierarchy frame.
 3. The method according to claim 1, whereinsaid connection map has a number of said identifications no greater thana maximum number of said tributaries in said frame plus one for each ofsaid time slots not carrying said tributaries.
 4. The method accordingto claim 1, wherein said connection map has a number of saididentifications no greater than a maximum number of said tributaries insaid frame plus one for a transport overhead of said frame, plus one fora path overhead of said frame and plus one for each of at least onestuff pattern of said frame.
 5. The method according to claim 4, whereinno greater than 28 of said identifications are used for switching saidtributaries and one of said identifications is used to switch a unitconsisting of a transport overhead of said frame and a path overhead ofsaid frame.
 6. The method according to claim 1, further comprising thestep of: prohibiting switching of a first of said time slots within aparticular tributary of said tributaries to a second of said time slotswithin said particular tributary.
 7. The method according to claim 1,further comprising the step of: prohibiting switching of a stuffedpattern in a predetermined slot of said time slots.
 8. The methodaccording to claim 1, further comprising the step of: inserting apattern into one of said time slots in response to said read addressindicating a predetermined one of said identifications.
 9. The methodaccording to claim 1, wherein 28 of said identifications are associatedwith said tributaries, a first one of said identifications is associatedwith four of said time slots carrying a transport overhead of said frameand a path overhead of said frame, a second one of said identificationsis associated with an alarm condition, a third one of saididentifications specifying that one of said time slots is unequipped anda fourth one of said identifications is associated with a predefinedconstant for signaling purposes.
 10. A system comprising: a switchcircuit configured to (i) buffer a particular frame comprising aplurality of time slots carrying a plurality of tributaries and (ii)switch said tributaries among said time slots in response a read signal;and a memory circuit configured to generate said read signal in responseto a plurality of identifications in a connection map stored therein,said connection map defining (i) at most one of said identifications foreach of said tributaries and (ii) one of said identifications for eachof said time slots carrying other than said tributaries.
 11. The systemaccording to claim 10, wherein said particular frame comprises one of aSynchronous Optical Network frame and a Synchronous Digital Hierarchyframe.
 12. The system according to claim 11, wherein said connection maphas less than 180QN┌log₂90PN┐ bits, where P is a number of input portsof said switch circuit, Q is a number of output ports of said switchcircuit and N is a number of either (i) SONET Synchronous TransportSignal, level-1 frames or (ii) Synchronous Transport Module, level-0frames in said particular frame.
 13. The system according to claim 12,wherein said connection map has no greater than 68QN┌log₂34PN┐ bits. 14.The system according to claim 12, wherein said connection map has nogreater than 64QN┌log₂32PN┐ bits.
 15. The system according to claim 12,wherein said connection map has no greater than 60QN┌log₂30PN┐ bits. 16.The system according to claim 10, wherein said memory circuit is furtherconfigured to store data in a tributary module specifying said frame asone of (i) a Synchronous Optical Network frame and (ii) a SynchronousDigital Hierarchy frame for use in switching each of said tributaries asa single unit.
 17. The system according to claim 10, wherein said memorycircuit is further configured to store data specifying said frame as oneof (i) a Synchronous Optical Network frame having a virtual tributarystructure, (ii) a Synchronous Digital Hierarchy frame having a virtualcontainer-3 structure and (iii) said synchronous Digital Hierarchy framehaving a virtual container-4 structure for use in switching each of saidtributaries as a single unit.
 18. The system according to claim 10,wherein said memory circuit is further configured to store dataspecifying (A) said frame as one of (i) a Synchronous Optical Networkframe and (ii) a Synchronous Digital Hierarchy frame and (B) said frameas one of (i) a Synchronous Optical Network frame having a virtualtributary structure, (ii) a Synchronous Digital Hierarchy frame having avirtual container-3 structure and (iii) said synchronous DigitalHierarchy frame having a virtual container-4 structure for use inswitching each of said tributaries as a single unit.
 19. The systemaccording to claim 10, further comprising: a logic block configured togenerate a pattern signal and a selection signal in response to saidread signal; and a multiplexer configured to multiplex said patternsignal and an output signal of said switch circuit in response to saidselection signal.
 20. A system comprising: means for buffering aplurality of tributaries disposed among a plurality of time slots in aframe; means for switching said tributaries among said time slots inresponse to a read address; and means for generating said read addressin response to a plurality of identifications in a connection map, saidconnection map defining (i) at most one of said identifications for eachof said tributaries and (ii) one of said identifications for each ofsaid time slots carrying other than said tributaries.